1. Field of the Invention
This invention relates to a semiconductor device having a silicon nitride film which forms an inter-layer film or a protective film designed to transmit ultraviolet rays. This invention also relates to a method of fabricating such a semiconductor device. One example of the semiconductor device of this invention is an ultraviolet erasable ROM (read-only memory) or an EPROM (an electrically programmable read-only memory or an erasable programmable read-only memory).
2. Description of the Prior Art
Some ultraviolet erasable ROM's use a silicon nitride film as a protective insulating film. In general, a silicon nitride film is formed in a reaction chamber by a plasma enhanced CVD (chemical vapor deposition) process.
Japanese published examined patent application 63-53703 discloses a semiconductor device having a silicon nitride film designed to transmit ultraviolet rays. In Japanese application 63-53703, a composition ratio "Si/N" (that is, an atomic ratio between silicon and nitrogen) of the silicon nitride film is preferably in the range of 0.65 to 0.825 to make the silicon nitride film sufficiently transparent with respect to ultraviolet rays having wavelengths equal to or shorter than 300 nm.
Japanese published unexamined patent application 3-129734 reports that a silicon nitride film having a composition ratio "Si/N" in the range of 0.65 to 0.825 tends to exert a great stress on a base or an under layer. Japanese application 3-129734 teaches a second insulating layer made of SiO.sub.2 which is provided between the silicon nitride film and the base.
Japanese Journal of Applied Physics, Vol 50, No. 6, pp. 638-649, 1981, indicates "Plasma CVD Technology and Its Application" which shows that a silicon nitride film made by a plasma enhanced CVD process has a great internal stress. Such a silicon nitride film inevitably contains hydrogen atoms. Further, it is shown that the strength of the internal stress depends on film growth conditions such as a composition ratio "Si/N" and a hydrogen atom concentration. Also, it is shown that an internal stress of a compression type weakens as a hydrogen atom concentration increases.
Japanese published unexamined patent application 6-267941 discloses a device protecting film including an ultraviolet transmissible SiN film. In Japanese application 6-267941, the SiN film is formed by a plasma enhanced CVD process. The SiN film has a composition ratio "Si/N" which preferably falls within the range of 0.75 to 0.87. An Si--H bond concentration "Z" (cm.sup.-3) in the SiN film is approximately expressed by the following formula. EQU Z=1.58.times.10.sup.22 X-9.94.times.10.sup.21
where "X" denotes the composition ratio "SiN". Further, a hydrogen bond concentration "Y" (cm.sup.-3) in the SiN film which determines the Si--H bond concentration is approximately expressed by the following formula. EQU Y=1.01.times.10.sup.22 X+0.54.times.10.sup.22
In addition, Japanese application 6-267941 shows that the SiN film transmits ultraviolet rays having a wavelength of 254 nm, and that the SiN film has a reduced internal stress and a high moisture resistance.